In an internal combustion engine system having D-EGR (dedicated exhaust gas recirculation), one or more cylinders of the engine are dedicated to operate in a rich combustion mode. Because of the rich combustion, the exhaust gases from the dedicated cylinder(s) have increased levels of hydrogen and carbon monoxide. Rich combustion products such as these are often termed “syngas” or “reformate”.
D-EGR engines use the reformate produced by the dedicated cylinder(s) in an exhaust gas recirculation (EGR) system. The hydrogen-rich reformate is ingested into the engine for subsequent combustion by the non-dedicated cylinders and optionally by the dedicated cylinder(s). The reformate is effective in increasing knock resistance and improving dilution tolerance and burn rate. This allows a higher compression ratio to be used with higher rates of EGR and reduced ignition energy, leading to higher efficiency and reduced fuel consumption.
For spark ignited engines, D-EGR and others, to address high engine pumping/throttling losses at idle and low loads, various cylinder deactivation (CDA) strategies have come into use. By shutting off all intake and exhaust valves to a subset of cylinders, and cutting off their fuel supply, the inducted fresh charge volume can be reduced without using the engine throttle. As an example, an inline four-cylinder engine with two deactivated cylinders (50% CDA) can operate at approximately half the engine load without having to throttle the fresh charge compared to a non-CDA engine. Because the deactivated cylinders act as an “air spring” the majority of the compression work is regained during the expansion stroke. As a result, pumping work is greatly reduced at part loads, leading to improved efficiency. Furthermore, to achieve a given engine torque, the non-deactivated cylinders must operate at a higher indicated mean effective pressure (IMEP). For instance, if 50% CDA is being used, the non-deactivated cylinders operate at twice the IMEP as the same engine without cylinder deactivation. At non-knock limited idle and part load conditions, greater IMEPs improve combustion efficiency, burn rates, combustion stability as well as emissions.
A conventional approach to achieving cylinder deactivation is to disable the intake and exhaust valves of the deactivated cylinders. This may be accomplished by disabling valve lift, such as by using a “lost motion” valvetrain with a cam lobe on a round base circle. This variability of valve action requires sophisticated valvetrains with hydraulic, electric or pneumatic actuators to switch between different cam lobes. Another approach is to disable the piston motion of individual cylinders by use of complicated mechanisms.